In my case?

1> Geometry
2> Lack of braze-ons
3> Tight clearances
4> Proper track dropouts.

Actually, my ideal frame would be a more like a path/6-day racer type frame, but that's probably more along the custom route than I'd like to go. The Flying Scot on eBay right now would be optimal, but i'm guessing it goes for around $800, less than I paid for my last 4 frames combined (two road, two track.)